History is littered with examples of byelection sensations that soon turned to dust. But Sarah Teather's victory in Brent East confirms the underlying trend that it is the Liberal Democrats rather than the Tories who are making the most of Labour's difficulties.

The catastrophic 29% drop in Labour's vote in Brent East since the general election may well be regarded as a direct verdict on Tony Blair's personal leadership and the government's record on public services. But that verdict must be tempered by the presence of a particularly large Muslim community for whom Iraq was a more important factor.

The scale of the Liberal Democrat achievement should not be underestimated, however. The swing - 29% from Labour to Liberal Democrat and 15% Conservative to Liberal Democrat - is one of the biggest turnarounds in British electoral history, albeit on a much lower turnout than most 20th century byelections.

It is also the first time that the Liberal Democrats have taken a byelection seat from Labour since Rosie Barnes's rabbits helped her to win Greenwich in 1987. To find another example you have to go back even further to Simon Hughes's victory in Bermondsey - a seat which he has held ever since, proving that not all Liberal Democrat triumphs turn to tears at the next general election.

Indeed, in the last decade the Lib Dems have hung onto three of their four byelection triumphs - in Newbury, Eastleigh and Romsey - at the subsequent general elections.

But the more comforting thought for Charles Kennedy this weekend - apart from the fact that he, too, was also once the youngest MP in the House of Commons - is that Brent East confirms that the underlying story in British politics since the last election has been the advance of his party at the expense of Labour.

The Tories were squeezed into a humiliating third place partly by widespread tactical voting against the government. The Tories had previously always been the very clear second place challengers in Brent East, particularly when Ken Livingstone ruled the local roost.

The Lib Dems took their highest ever share of the vote, 30%, in this year's local elections and now have 54 MPs, their highest total since 1929. Their opinion poll ratings since the general election have consistently moved up from an average of 19% to 22% today while the Tories continue to stay stuck around the 32 point mark.

The problem is compounded for the Conservatives by the distribution of the key battleground marginal seats at the next general election. They are far more vulnerable to a Liberal Democrat advance than Labour. This is because they face a Lib Dem second place challenger in seven out of their 15 most marginal seats. In contrast, the Lib Dems are second in only one of Labour's 25 most marginal seats - Cardiff Central.

The Tories paid the price for this at the last general election and could do so again at the next. In 2001 they gained nine seats - two of them from the Liberal Democrats, the Isle of Wight and Taunton. But at the same time they lost eight seats - seven of them to the Liberal Democrats.

There are a few seats, such as South Dorset and Braintree, where the Liberal Democrats are in third place and a third party revival would help the Conservatives to regain the seats lost to Labour but they are outnumbered by vulnerable Tory marginals.

The last time this effect of the first past the post system was seen was in the 1980s during the heyday of the Liberal-SDP Alliance, when Simon Hughes and Rosie Barnes won famous victories. Those byelection triumphs ended with Margaret Thatcher celebrating her third term victory in 1987. The underlying lesson from Brent East may well prove to be that the revival in the fortunes of the third party can help a troubled incumbent government to stay in power.